This invention relates to a novel process for the production of complexes of platinum and organosiloxanes and to the use of said complexes.
It is well known in the art of organosilicon chemistry that organosilicon compounds having silicon-bonded hydrogen atoms can be reacted with organic or organosilicon compounds having ethylenic unsaturation in the presence of a platinum compound as catalyst. The use of chloroplatinic acid as such a catalyst has been disclosed by Hook et al. in U.S. Pat. No. 2,823,218.
Willing, in U.S. Pat. No. 3,419,593 disclosed that the compatibility of chloroplatinic acid in organosilicon reaction mixtures can be improved by reacting chloroplatinic acid with an organosilicon compound, preferably an organosiloxane, having silicon-bonded unsaturated groups. A method for preparing platinum complexes of unsaturated siloxanes which are useful as hydrosilylation catalysts was disclosed by Karstedt in U.S. Pat. No. 3,775,452. According to this method an unsaturated organosilicon material is mixed with a platinum halide, typically chloro-platinic acid or sodium or potassium chloroplatinate, and the resulting mixture is then treated with a base to effect the removal of inorganic halogen. Such prior art methods, however, require the presence of water, a very large excess of the vinylsiloxane or long reaction times. The use of a large excess of the vinylsiloxane represents a significant drawback to these type of methods inasmuch as some loss of this relatively expensive reactant occurs as a result of cleavage of the vinyl groups and any remaining excess has to be recovered from the reaction mixture.
Further, it has been discovered that the presence of water contributes to the loss of vinyl groups from the siloxane reactant and increases the need for an excess of the vinylsiloxane. It was suggested by Karstedt in U.S. Pat. No. 3,775,452 (Col. 7, line 40) that the complex can be recovered in admixture with the excess vinylsiloxane and employed as such. However, the presence of the vinylsiloxane can reduce the activity of the platinum as a catalyst. Also, such an impure form of the complex may not be satisfactory for certain applications.
It has been discovered by the present inventors that by employing specific platinum compounds, namely platinous halides, complexes of platinum and organosiloxanes can be obtained by a method which does not require the presence of large excesses of the organosiloxane or of water.
It was suggested by Kelly et al. in U.S. Pat. No. 3,474,123 that complexes of platinous chloride and unsaturated organosiloxanes can be prepared by direct reaction, as in the case of organic complexes of platinum. However, the yields obtained by such a direct reaction are very low and it should be noted that all of the Examples in the '123 patent illustrate the alternative method described therein which involves the additional step of first forming the organic complex.
Brown et al. in U.S. Patent No. 5,175,325 discloses a method for preparing a platinum-organosiloxane complex which comprises reacting a platinous halide with an organosiloxane in which there are present from 2 to 4 silicon-bonded organic groups having from 2 to 6 carbon atoms and terminal olefinic unsaturation, the remaining silicon-- bonded organic substituents being selected from alkyl groups having from 1 to 6 carbon atoms and phenyl groups, said reaction being carried out in the presence of a polar organic liquid which is at least a partial solvent for the platinous halide. However, nowhere in Brown et al. is it disclosed that the crystal size of the platinous halide has any effect on the reaction rate of the method of their invention.